by John McPhee
What called most for demonstration was Hutton’s essentially novel and all but incomprehensible sense of time. In 4004 + 1785 years, you would scarcely find the time to make a Ben Nevis, let alone a Gibraltar or the domes of Wales. Hutton had seen Hadrian’s Wall running across moor and fen after sixteen hundred winters in Northumberland. Not a great deal had happened to it. The geologic process was evidently slow. To accommodate his theory, all that was required was time, adequate time, time in quantities no mind had yet conceived; and what Hutton needed now was a statement in rock, a graphic example, a breath-stopping view of deep time. There was a formation of “schistus” running through southern Scotland in general propinquity to another formation called Old Red Sandstone. The schistus had obviously been pushed around, and the sandstone was essentially flat. If one could see, somewhere, the two formations touching each other with strata awry, one could not help but see that below the disassembling world lie the ruins of a disassembled world below which lie the ruins of still another world. Having figured out inductively what would one day be called an angular unconformity, Hutton went out to look for one. In a damp country covered with heather, with gorse and bracken, with larches and pines, text-book examples of exposed rock were extremely hard to find. As Hutton would write later, in the prototypical lament of the field geologist, “To a naturalist nothing is indifferent; the humble moss that creeps upon the stone is equally interesting as the lofty pine which so beautifully adorns the valley or the mountain: but to a naturalist who is reading in the face of rocks the annals of a former world, the mossy covering which obstructs his view, and renders undistinguishable the different species of stone, is no less than a serious subject of regret.” Hutton’s perseverance, though, was more than equal to the irksome vegetation. Near Jedburgh, in the border country, he found his first very good example of an angular unconformity. He was roaming about the region on a visit to a friend when he came upon a stream cutbank where high water had laid bare the flat-lying sandstone and, below it, beds of schistus that were standing straight on end. His friend John Clerk later went out and sketched for Hutton this clear conjunction of three worlds—the oldest at the bottom, its remains tilted upward, the intermediate one a flat collection of indurated sand, and the youngest a landscape full of fences and trees with a phaeton-and-two on a road above the rivercut, driver whipping the steeds, rushing through a moment in the there and then. “I was soon satisfied with regard to this phenomenon,” Hutton wrote later, “and rejoiced at my good fortune in stumbling upon an object so interesting to the natural history of the earth, and which I had been long looking for in vain.”
What was of interest to the natural history of the earth was that, for all the time they represented, these two unconforming formations, these two levels of history, were neighboring steps on a ladder of uncountable rungs. Alive in a world that thought of itself as six thousand years old, a society which had placed in that number the outer limits of its grasp of time, Hutton had no way of knowing that there were seventy million years just in the line that separated the two kinds of rock, and many millions more in the story of each formation—but he sensed something like it, sensed the awesome truth, and as he stood there staring at the riverbank he was seeing it for all humankind.
To confirm what he had observed and to involve further witnesses, he got into a boat the following spring and went along the coast of Berwickshire with John Playfair and young James Hall, of Dunglass. Hutton had surmised from the regional geology that they would come to a place among the terminal cliffs of the Lammermuir Hills where the same formations would touch. They touched, as it turned out, in a headland called Siccar Point, where the strata of the lower formation had been upturned to become vertical columns, on which rested the Old Red Sandstone, like the top of a weather-beaten table. Hutton, when he eventually described the scene, was both gratified and succinct—“a beautiful picture … washed bare by the sea.” Playfair was lyrical:
On us who saw these phenomena for the first time, the impression made will not easily be forgotten. The palpable evidence presented to us, of one of the most extraordinary and important facts in the natural history of the earth, gave a reality and substance to those theoretical speculations, which, however probable, had never till now been directly authenticated by the testimony of the senses. We often said to ourselves, What clearer evidence could we have had of the different formation of these rocks, and of the long interval which separated their formation, had we actually seen them emerging from the bosom of the deep? We felt ourselves necessarily carried back to the time when the schistus on which we stood was yet at the bottom of the sea, and when the sandstone before us was only beginning to be deposited, in the shape of sand or mud, from the waters of a superincumbent ocean. An epocha still more remote presented itself, when even the most ancient of these rocks, instead of standing upright in vertical beds, lay in horizontal planes at the bottom of the sea, and was not yet disturbed by that immeasurable force which has burst asunder the solid pavement of the globe. Revolutions still more remote appeared in the distance of this extraordinary perspective. The mind seemed to grow giddy by looking so far into the abyss of time.
Hutton had told the Royal Society that it was his purpose to “form some estimate with regard to the time the globe of this Earth has existed.” But after Jedburgh and Siccar Point what estimate could there be? “The world which we inhabit is composed of the materials not of the earth which was the immediate predecessor of the present but of the earth which … had preceded the land that was above the surface of the sea while our present land was yet beneath the water of the ocean,” he wrote. “Here are three distinct successive periods of existence, and each of these is, in our measurement of time, a thing of indefinite duration … . The result, therefore, of this physical inquiry is, that we find no vestige of a beginning, no prospect of an end.”
The Old Red Sandstone was put down by rivers flowing southward to a sea where marine strata were accumulating in the region that is now called Devon. The size, speed, and direction of the rivers—their islands, pitches, and bends—are not just inferable but can almost be seen, in structures in the Old Red Sandstone: gravel bars, point bars, ripples of the riverbeds, migrating channels, “waves” that formed of sand. The sea into which those rivers spilled ran all the way to Russia, but it was in the rock of Devonshire that geologists in the eighteen-thirties found cup corals—fossilized skeletons, cornucopian in shape—that were not of an age with corals they had found before. They had found related corals that were obviously less developed than these, and they had found corals that were more so. The less developed corals had been in rock that lay under the Old Red Sandstone. The more developed corals had been in rock above the Old Red Sandstone. Therefore, it was inferred (correctly) that the Old Red Sandstone of North Britain and the marine limestone of Devon were of the same age, and that henceforth any rock of that age anywhere in the world—in downtown Iowa City; on Pequop Summit, in Nevada; in Stroudsburg, Pennsylvania; in Sandusky, Ohio—would be called Devonian. It was a name given, although they did not know it then, to forty-six million years. They still had no means of measuring the time involved. They also had no way of knowing that those forty-six million years had ended a third of a billion years ago. All they had was their new and expanding insight that they were dealing with time in quantities beyond comprehension. Devonian—408 to 362 million years before the present.
Geologists did not have to look long at the coal seams of Europe—the coals of the Ruhr, the coals of the Tyne—to decide that the coals were of an age, which they labelled Carboniferous. The coal and related strata lay on top of the Old Red Sandstone. So, in the succession of time, the Carboniferous period (eventually subdivided into Mississippian and Pennsylvanian in the United States) would follow the Devonian, coupling on, as the science would eventually determine, another seventy-two million years—362 to 290 million years before the present.
In this manner—with their fossil assemblages and faunal successions,
their hammers decoding rock—geologists in the first eighty years of the nineteenth century constructed their scale of time. It was based on the irreversible history of life. Crossing the century, it both anticipated and confirmed Darwin. When the Devonian was defined in the light of the changes in corals, Darwin was obscure and not long off the Beagle, with twenty years to go before The Origin of Species. Meanwhile, the geologists were out correlating strata and reading there a record less of rock than of life. The rock had been recycled, and sandstones of one era could be indistinguishable from the sandstones of another, but evolution had not occurred in cycles, so it was through the antiquity of fossils that geologists worked out the comparative ages of the rock in which the fossils were preserved. Some creatures were more useful than others. Oysters and horseshoe crabs, for example, were of marginal assistance. Oysters had appeared in the Triassic, horseshoe crabs in the Cambrian. Both had evolved minimally and had obviously avoided extinction. Some creatures, on the other hand, had appeared suddenly, had evolved quickly, had become both abundant and geographically widespread, and then had died out, or died down, abruptly. Geologists canonized them as “index fossils” and studied them in groups. Experience proved that the surest method of working out relative ages of rock was not through individual creatures but through the relating of successive strata to whole collections of creatures whose fossils were contained therein—a painstaking comparison of arrivals and extinctions that helped to characterize the divisions of the time scale and define its boundaries with precision.
Imagine an E. L. Doctorow novel in which Alfred Tennyson, William Tweed, Abner Doubleday, Jim Bridger, and Martha Jane Canary sit down to a dinner cooked by Rutherford B. Hayes. Geologists would call that a fossil assemblage. And, without further assistance from Doctorow, a geologist could quickly decide—as could anyone else—that the dinner must have occurred in the middle eighteen-seventies, because Canary was eighteen when the decade began, Tweed became extinct in 1878, and the biographies of the others do not argue with these limits. In progressive refinements, geologists with their fossil assemblages established their systems and series and stages of rock, their eras and periods and epochs of time. But, unlike Doctorow, who deals with a mere half-dozen people around a dinner table, the geologists would assemble from one set of strata hundreds and even thousands of species from all over the food chain, and by lining up their genetic histories side by side establish with near-certainty points in comparative time.
Some of these time lines were bolder than others, and none more so than the one that underlined the first appearance of megascopic fossils in abundance in the world. It marked a great and sudden explosion of life, all the major phyla having developed more or less at the same time and now acquiring skeletons and shells and teeth and other hard components that allowed them individually to be reported to the future. Because rock that held these early fossils was first studied on Harlech Dome and adjacent Welsh terrains, geologists named the system Cambrian, after the Roman name for Wales. They then named the Silurian for a Welsh tribe that bitterly defied the Romans. After some years and more comparative study, an argument broke out over the Cambro-Silurian line, a scientific battle royal in which the Cambrian forces tried to move their banner forward through time and the Silurian proponents attempted to push theirs back. The disputed block of time became a sort of demilitarized zone. Friendships came unstuck. The standoff lasted for decades, until some genius in scientific diplomacy suggested that the disputed time had enough characteristics of its own to be given the status of a discrete period, an appropriate name for which—in honor of another tribe of intractable Welsh belligerents—would be Ordovician. There was a lot of room for generosity. There was plenty of time for all. Cambrian—544 to 490. Ordovician—490 to 439. Silurian—439 to 408 million years before the present.
A British geologist went to Russia and after a season or two’s tapping at the Urals named still another period in time, and system of rock, for the upland oblast of Perm. There were formations in Perm with a fossil story distinctly their own that were superimposed—as they happen to be in Pennsylvania, as they happen to be at the rim of the Grand Canyon—upon the Carboniferous. What was distinct about the character of the Permian assemblages was not only the forms to which they had evolved but also their absence in great numbers from higher, younger strata. There had evidently been a wave of death, in which thousands of species had vanished from the world. No one has explained what happened—at least not to the general satisfaction. A drastic retreat of shallow seas may have destroyed innumerable environments. The cause may have been extraterrestrial—lethal radiation from a supernova dying nearby. The wave of death occurred 250.1 million years before the present, and exactly that long ago flood basalts emerged in Siberia and quickly covered about a million and a half square kilometres with incandescent lava. The brief, intense greenhouse effect, the surge of carbondioxide emissions, would have stopped the upwelling of the oceans and the associated growth of nutrients. None of these hypotheses has attracted enough concurrence to be dressed out in full as a theory, but, whatever the cause, no one argues that at least half the fish and invertebrates and three-quarters of all amphibians—perhaps as much as ninety-six per cent of all marine faunal species—disappeared from the world in what has come to be known as the Permian Extinction.
It was an extinction of a magnitude that would be approached only once in subsequent history, or—to express that more gravely —only once before the present day. The sharp line of creation at the outset of the Cambrian had an antiphonal parallel in the Permian Extinction, and the whole long stretch between the one and the other was set apart in history as the Paleozoic era. It was a unit—well below the surface but far above the bottom—just hanging there suspended in the formless pelagics of time. The Paleozoic—544 to 250 million years before the present, a fifteenth of the history of the earth. Cambrian, Ordovician, Silurian, Devonian, Mississippian, Pennsylvanian, Permian. When I was seventeen, I used to accordion-pleat those words, mnemonically capturing the vanished worlds of “Cosdmpp,” the order of the periods, the sequence of the systems. It was either that or write them in the palm of one hand.
Lyell, Cuvier, Conybeare, Phillips, von Alberti, von Humboldt, Desnoyers, d’Halloy, Sedgwick, Murchison, Lapworth, Smith (William “Strata” Smith): the geologists who extended Hutton’s insight and built this time scale conjoined their names in the history of the science in a way that would not be repeated for more than a hundred years, until a roster of comparable length—Hess, Heezen, McKenzie, Morgan, Wilson, Matthews, Vine, Parker, Sykes, Ewing, Le Pichon, Cox, Menard—would effect the plate-tectonics revolution. The system of rock immediately above the Paleozoic, in which all that Permian life failed to reappear, was typified by three formations in Germany—certain sandstones, limestones, and marly shales—that ran like a striped flag through the Black Forest, the Rhine Valley, and lent the name Triassic to forty-two million years. In the Triassic, the earliest subdivision of the Mesozoic era, two families of reptiles that had survived the Permian Extinction began to show patterns of unprecedented growth. This would continue for a hundred and fifty million years—through the Jurassic and out to the end of Cretaceous time, when the “fearfully great lizards,” on the point of disappearance, would reach their greatest size, not to be surpassed until epochs that followed the Eocene development of whales. European geologists studying the massive limestones of the Jura—the gentle mountains of the western cantons of Switzerland and of Franche-Comte—related the copious displays of ancient life there to comparable assemblages elsewhere in the world, and called them all Jurassic. A primordial bird appeared in the Jurassic. It had claws on its wings and teeth in its bill and a reptile’s long tail sprouting feathers. Its complete performance envelope as a flier was to climb a tree and jump.
Physicists, chemists, and mathematicians, taking note of all the nomenclatural inconsistencies—of time named for mountain ranges, time named for savage tribes, time named for a country here, a co
unty there, an oblast in the Urals—have politely, gently, suggested that, in this one sense only, the time scale seems archaic, seems, if one may say so, out of date. Geology might be better served by a straightforward system of numbers. The reaction of geologists, by and large, has been to look upon this suggestion as if it had come over a bridge that exists between two cultures. A Continental geologist, in 1822, named eighty million years for the white cliffs of Dover, for the downs of Kent and Sussex, for the chalky ground of Cognac and Champagne. Related strata were spread out through Holland, Sweden, Denmark, Germany, and Poland. He called it Le Terrain Cretace. If that name was apt, his own was irresistible. He was J. J. d‘Omalius d’Halloy. Triassic, Jurassic, Cretaceous. When the Cretaceous ended, the big marine reptiles had disappeared, the flying reptiles, the dinosaurs, the rudistid clams, and many species of fish, not to mention the total elimination or severe reduction of countless smaller species from the sea. At the same point in geologic time, the flood basalts now known as the Deccan Traps came out of the mantle and quickly covered at least a million square kilometres in India, effectively stopping the upwelling of the ocean. An ocean gone stagnant would kill phytoplankton, which prosper in the currents of mixed-up seas. Break the food chain and creatures die out above the break. Phytoplankton are the base of the food chain. The Arctic Ocean, surrounded by continents that had drifted together, might have become in the Cretaceous the greatest lake in all eternity, and when the North Atlantic opened up enough to let the water flood the southern seas the life in them would have suffered a cold osmotic shock. Drastic fluctuations of sea level—also related, perhaps, to the separation of continents—might have caused changes in air temperature and ocean circulation that were enough to sunder the food chain. At the end of 1979, a small group at the Lawrence Radiation Laboratory, in Berkeley—among them the physicist Luis Alvarez, winner of a Nobel Prize, and his son Walter, who is a geologist—brought forth a piece of science in which they presented the catastrophe as the effect of an Apollo Object colliding with the earth. An Apollo Object is an “earth-orbit-crossing” asteroid that is at least a kilometre in diameter and is in the category of asteroids that have pockmarked the surface of Mercury, Mars, and the moon, and the surface of the earth as well, although most of the evidence has been obscured here by erosion. Like the general run of meteorites, an Apollo Object could be expected to contain a percentage of iridium and other platinum-like metals at least a thousand times greater than the concentration of the same metals in the crust of the earth. In widely separated parts of the world—Italy, Denmark, New Zealand—the Berkeley researchers found a thin depositional band, often just a centimetre thick, that contains unearthly concentrations of iridium. Below that sharp line are abundant Cretaceous fossils, and above it they are gone. It marks precisely the end of Cretaceous time. The Berkeley calculations suggested an asteroid about six miles in diameter hitting the earth with a punch of a hundred million megatons, making a crater a hundred miles wide. Such an occurrence—which could repeat itself tomorrow afternoon, there being several hundred big asteroids out there in threatening orbits —would have sent up a mushroom cloud containing some thirty thousand cubic kilometres of pulverized asteroid and terrestrial crust, part of which would have gone into the stratosphere and spread quickly over the earth, keeping sunlight off the lands and seas and suppressing photosynthesis. A decade after the publication of the Berkeley hypothesis, Chicxulub Crater was discovered, buried five hundred metres under Yucatan. Evidently made by an Apollo Object, it is a hundred and ten miles wide. On August 26 and 27, 1883, when the island Krakatoa, in the Sunda Strait, exploded with great violence, it sent less than twenty cubic kilometres of material into the air, but within a few days dust had spread above the whole earth, turning daylight into dusk. It made exceptionally brilliant sunsets for two and a half years. Edmund Halley, who died when James Hutton was fifteen, once wrote a paper suggesting that the way God started Noah’s Flood was by directing a big comet into collision with the earth. The Cretaceous Extinction, whatever its cause, was one of the two most awesome annihilations of life in the history of the world. With the Permian Extinction before it, it framed the Mesozoic, an era of burgeoning creation within deadly brackets of time.
